Methods of operating a series hybrid vehicle

ABSTRACT

The invention is directed toward methods for operating a series hybrid vehicle in a manner that responds to the operator&#39;s demand for power output, while maximizing engine efficiency and minimizing disruptions in vehicle drivability. According to principles of the present invention, when the driver of a series hybrid vehicle makes a demand for power output, whether the secondary power source(s) is supplied with secondary energy stored in an energy storage device(s), direct input energy generated by an engine(s), or both, depends on the amount of available secondary energy stored in the vehicle&#39;s secondary storage device(s) alone, and in combination with vehicle speed. During the time that the engine is used to generate secondary energy, the power efficiency level at which the engine is operated also depends on the vehicle speed and the amount of available secondary energy stored in the vehicle&#39;s secondary storage device alone, and in combination with vehicle speed. Further, in some embodiments, when the engine is not generating secondary energy, the engine is selectively turned off or idled in response to various operating conditions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to methods for operating aseries hybrid vehicle and, more specifically, to methods for maximizingfuel efficiency while minimizing disruptions in drivability.

2. Description of the Related Art

The term “hybrid vehicle,” in the broadest sense, denotes a vehiclehaving more than one power source and one or more energy storage means.The goal of a hybrid vehicle is to combine several similar or dissimilartypes of energy stores and/or energy converters with different drivecomponents, and operate each power source under varying operatingconditions in a manner that results in greater overall energy savingsthan would otherwise be achieved through the use of a single powersource.

The primary power source of a hybrid vehicle is usually an enginepowered by fuel energy (primary energy), and the secondary power sourceis usually, but not limited to, one or more electric motors/generatorspowered by electric energy (a form of “secondary energy”) and/or one ormore hydraulic motors/pumps powered by hydraulic pressure (also a formof “secondary energy”).

When the drive components of a hybrid vehicle allow the vehicle'sprimary and secondary power sources to both independently transmit powerto the vehicle's wheels, the vehicle is commonly referred to as aparallel hybrid vehicle and the wheels of the vehicle can be drivensolely by an engine (as is done with conventional vehicles), or solelyby the secondary power source. In contrast, when the drive components ofa hybrid vehicle are configured such that only the vehicle's secondarypower source transmits power to the vehicle's wheels, the vehicle iscommonly referred to as a series hybrid vehicle. In series hybridvehicles, the engine is used to convert energy and provide power withwhich to power the secondary power source, but the engine is notmechanically linked to the vehicle's wheels.

To date, parallel hybrid vehicles have been more commercially successfulthan series hybrid vehicles. For example, the Insight, a hybrid vehiclemanufactured by Honda Motor Company, and the Prius, a hybrid vehiclemanufactured by Toyota Motor Corporation, represent the first twomass-marketed hybrids, and both are parallel hybrid vehicles. Thecommercial success of parallel hybrid vehicles over series hybrids is,in large part, due to the state of technology and knowledge that havebeen available with respect to energy storage devices used for storing ahybrid vehicle's secondary energy. For example, many of the firstgeneration secondary energy storage devices, such as early generationbatteries, require a low charge rate in order to preserve the life ofthe energy storage device. This low charge rate requirement restrictsthe design choices available to a hybrid vehicle designer and, inparticular, restricts the choices available for a series hybrid morethan it restricts the choices available for a parallel hybrid. In serieshybrid vehicles, the charge rate is, by definition, provided by anengine. Thus, design choices affecting the size and calibration of anengine in a series hybrid vehicle employing previous generation energystorage devices are limited by the need to have the engine of a serieshybrid produce a low enough power level to generate the required lowcharge rate, while still achieving greater overall energy savings fromthe hybrid design than would otherwise be achieved through the use of asingle power source.

Since engine efficiency is better at high loads than at low loads,engines in prior art series hybrid vehicles are typically very small,and are calibrated to operate at high loads. This allows the engine tooperate closer to its maximum efficiency level while still producing alow enough power level to generate the required low charge rate.However, due to the low charge rate, the energy stored within previousgeneration energy storage devices is often used up more quickly than itcan be replenished. Thus, when the energy stored within the energystorage device of a series hybrid vehicle is depleted, the driver isunable to complete a trip because the engine alone is too small tosafely propel the vehicle.

As a result, there is a need for a new and improved method of operatinga series hybrid vehicle.

BRIEF SUMMARY OF THE INVENTION

The invention is directed toward new and improved methods for operatinga series hybrid vehicle in a manner designed to further the vehicle'soverall energy efficiency gains.

According to principles of the present invention, when the driver of aseries hybrid vehicle makes a demand for power output, a secondary powersource(s) is supplied with, and thereby powered by, either (1) secondaryenergy stored in an energy storage device(s), (2) secondary energygenerated by an engine(s) and used to directly supply power to thesecondary power source (“direct input energy”, or (3) both. Thedetermination as to which selection is made depends on the amount ofavailable secondary energy stored in the vehicle's secondary energystorage device(s), and in some cases depends also on the power levelbeing demanded by the driver. If the engine is not generating secondaryenergy, the engine is either turned off or at idle. However, if theengine is generating secondary energy, the power/efficiency level atwhich the engine operates depends on either (1) the amount of availablesecondary energy stored in the vehicle's secondary storage device, or(2) the amount of available secondary energy stored in the vehicle'ssecondary storage device and the vehicle speed.

In one embodiment, a series hybrid vehicle is operated by selectivelygenerating an amount of primary power from a primary power source,converting a first portion of the amount of primary power from theprimary power source into an amount of direct input energy, and poweringthe secondary power source directly with the amount of direct inputenergy.

In another embodiment, a secondary power source in a series hybridvehicle is operated by monitoring an amount of available stored energywithin an energy storage device and operating an engine (1) at or near afirst power level when the amount of available stored energy is within apredetermined upper range of available stored energy, (2) at or near asecond power level when the amount of available stored energy is withina predetermined lower range of available stored energy, and (3) within arange of power levels when the amount of available stored energy iswithin a predetermined middle range of available stored energy.

In yet another embodiment, a series hybrid vehicle is operated bymonitoring an amount of available stored energy within an energy storagedevice and, based on the amount of available stored energy, selectivelypowering the secondary power source with either (1) a portion of theamount of available stored energy, (2) a portion of an amount of directinput energy, or (3) a combination of a portion of the amount ofavailable stored energy and a portion of the amount of direct inputenergy.

BRIEF DESCRIPTION OF THE SEVERAL FIGURES

FIG. 1 is a schematic diagram of a series hybrid vehicle provided inaccordance with the present invention.

FIG. 2 is a graphic representation for controlling the operation of aseries hybrid vehicle according to one embodiment of the presentinvention.

FIG. 3 is a logic flow diagram for controlling the operation of a serieshybrid vehicle used in accordance with the embodiment provided in FIG.2.

FIG. 4 is an exemplary power efficiency map for an engine in a serieshybrid vehicle, showing exemplary target power points at which theengine is operated when used in accordance with the embodiment providedin FIG. 2.

FIG. 5 is a logic flow diagram for controlling the operation of anengine in the series hybrid vehicle used in accordance with theembodiment provided in FIG. 2.

FIG. 6 is a graphic representation for controlling the operation of anengine in a series hybrid vehicle according to another embodiment of thepresent invention.

FIG. 7 (collectively shown as FIG. 7A and 7B) is a logic flow diagramfor controlling the operation of an engine in a series hybrid vehicleaccording to yet another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various embodiments of theinvention. However, one of ordinary skill in the art will understandthat the invention may be practiced without these details. In otherinstances, well-known structures associated with hybrid vehicles havenot been shown or described in detail to avoid unnecessarily obscuringdescriptions of the embodiments of the invention.

The term “primary power source,” as used herein, denotes an engine suchas an internal combustion engine (e.g., a compression ignition engine, aspark ignition engine, or gas turbine engine) or an external combustionengine (e.g., a Stirling engine), a fuel cell, or other primary energyconverter.

The term “variable displacement engine,” as used herein, refers to amulti-cylinder engine wherein each of the cylinders is selectivelyoperated (individually or as a group) such that the engine's totaldisplacement is thereby selectively increased or decreased.

The term “secondary power source,” as used herein, denotes a powersource having a two-way energy path and thus capable of capturing avehicle's kinetic energy during the vehicle's braking process. Asecondary power source may include, for example, one or more electric orhydraulic pump/motors. As is to be understood by one of ordinary skillin the art, other like systems may also be employed, and the secondarypump/motors described herein do not limit the scope of the invention.

Depending on the type of secondary power system selected for use, theenergy used to power the secondary power source (“secondary energy”) mayconsist of electric energy, hydraulic energy, or any other form ofenergy that can be, at least in part, obtained from the vehicle'skinetic energy during the braking process, and reused to power asecondary power source.

The term “energy storage device,” as used herein, denotes a systemcapable of receiving and storing the secondary energy, and allowing forits reuse to power a secondary power source. Such a system may, forexample, consist of ultracapacitors, electric batteries, mechanicalflywheels or hydraulic accumulators. As is to be understood by one ofordinary skill in the art, other like systems may also be employed, andthe systems described herein do not limit the scope of the invention.

The term “available stored energy,” as used herein, refers to all of theenergy stored in an energy storage device, less any minimal amount whichmay be necessary to maintain the functionality of the storage deviceand/or less any amount used to supply energy to a device other than asecondary power source used to propel the vehicle.

The term “direct input energy,” as used herein, refers to secondaryenergy generated by a primary power source and used to directly supplyenergy to a secondary power source, as opposed to storing the energy foruse at a later time.

The term “storable energy,” as used herein, refers to energy generatedby a primary power source or a regenerative braking system and capableof being stored within an energy storage device to power a secondarypower source at a later time.

Further, the terms “primary power source,” “secondary power source,”“engine,” “energy storage device,” “control processing unit,” and othercomponents of the present invention are, for ease of discussion, oftenreferred to herein in the singular. However, as will be understood byone of ordinary skill in the art, the present invention may employ morethan one of the components used to perform the functions of the presentinvention, and thus components referred to in the singular are not to beconstrued as limiting the number of components employed.

The headings provided herein are for convenience only and do not defineor limit the scope or meaning of the claimed invention.

Applicability and General Overview

According to principles of the present invention, when the driver of aseries hybrid vehicle 10 (FIG. 1) makes a demand for power output, asecondary power source(s) 12 is used to propel the vehicle. Thesecondary power source 12 is supplied with, and thereby powered by,either (1) an amount of available stored energy in an energy storagedevice(s) 14, (2) direct input energy generated by an engine(s) 16, or(3) both. The determination as to which selection is made depends on theamount of available stored energy stored within the vehicle's 10 energystorage device 14. When the engine 16 is used, the efficiency level atwhich the engine 16 operates depends on either (1) the amount ofavailable secondary energy stored in the vehicle's 10 energy storagedevice 14 or (2) the vehicle's 10 speed and the amount of availablesecondary energy stored in the vehicle's 10 energy storage device 14.

As shown in FIG. 1, the secondary power source 12, for example apump/motor, is coupled to the primary power source (engine) 16 via agenerator 28, for example a pump/motor. When the engine 16 is operating,the generator 28 is used to convert the engine's 16 power into energycompatible for input into the secondary power source (e.g., electriccurrent or pressurized hydraulic fluid). The converted energy is eithersupplied directly to the secondary power source 12 as direct inputenergy to power the secondary power source 12 as a motor, and/orsupplied to the vehicle's energy storage device 14 and stored for lateruse (storable energy). As is to be understood by one of ordinary skillin the art, the type of generator 28 used necessarily depends on thetype of energy required to operate the secondary power source 12. Forexample, if the secondary power source 12 is an electricgenerator/motor, then the generator 28 is an electric generator.Similarly, if the secondary power source 12 is a hydraulic pump/motor,then the generator 28 is a hydraulic pump. Generator 28 may also be usedto start the engine 16 by acting as a motor using energy from energystorage device 14.

Fuel energy stored in a vehicle tank (not shown) is used to power theengine 16. An engine control device 20, coupled to the engine 16, and incommunication with a CPU 18, controls fuel delivery to the engine 16. Agenerator control device 80, coupled to the generator 28, and incommunication with CPU 18, controls the speed of engine 16 by varyingload. Based on the amount of available stored energy and, optionally,the vehicle speed, the CPU 18 issues a command signal C_(S1) to theengine control device 20 and a command signal C_(S2) to the generatorcontrol device 80 to operate the engine 16 at a number of preselectedpower levels.

As is known to those of ordinary skill in the art, an engine 16 can beoperated at a preselected power level by operating the engine at apreselected engine speed for a given engine torque value. As is furtherknown to those of ordinary skill in the art, a desired engine torque canbe achieved by increasing or decreasing the amount of fuel supplied toan engine 16. Thus, included among the many sensors (not all shown)which provide an input signal I_(S) to the CPU 18 of the presentinvention, there are sensors which detect and monitor engine speed andengine torque. Other sensors detect the driver's command to brake thevehicle 10, the driver's command to power the vehicle 10, and monitorvehicle speed. For example, the driver's demand to power the vehicle isrepresented by throttle sensor 22.

Further, a secondary energy capacity sensor 24 monitors the amount ofavailable stored energy at any given time and generates a signal E_(s)representative of the energy detected. The CPU 18 also includes a memoryfor storing various lookup tables. Methods of monitoring an amount ofavailable stored energy and issuing commands in response to detecting apredetermined amount of available energy in a hybrid vehicle aredescribed in commonly assigned pending U.S. patent application Ser. No.10/386,029, filed Mar. 10, 2003, entitled “METHODS OF OPERATING APARALLEL HYBRID VEHICLE,” which is incorporated herein by reference.

A secondary power source control device 26 is coupled to the secondarypower source 12 and used to control operation of the secondary powersource 12. Thus, when a driver issues a command to power the vehicle 10,the CPU 18 detects this command and issues a command signal C_(S3)directing the secondary power source control device 26 to operate thesecondary power source 12 as a motor. When in motor mode, the secondarypower source 12 transmits power through a mechanical linkage 30 to thevehicle's 10 wheels 32, and thereby propels the vehicle 10.

As mentioned above and explained in further detail below, when theengine 16 is operating, an amount of energy from the engine 16 isconverted into an amount of storable energy and stored within thevehicle's energy storage device 14 when certain vehicle 10 operatingparameters are met. However, as is known to those of ordinary skill inthe art, storable energy can also be obtained by capturing the vehicle'skinetic energy during a braking event.

When a driver issues a command to brake the vehicle 10 and the amount ofavailable energy stored within the energy storage device 14 is eitherbelow full capacity or below a preselected level, the CPU 18 directs thesecondary power source control device 26 to operate the secondary powersource 12 as a generator/pump. The vehicle's kinetic energy is thendirected to the generator/pump 12, converted into an amount of storableenergy, and stored within the vehicle's 10 energy storage device 14.

Determining How To Power The Secondary Power Source

FIGS. 2 and 3 show one embodiment for supplying power to the secondarypower source 12 in response to a demand to power the vehicle. In thisembodiment, an amount of available stored energy within the vehicle's 10energy storage device 14 is monitored and if the available stored energyis at or above a first selected level of available stored energy(depicted as line 37 in FIG. 2, and at step 301 in FIG. 3), the primarypower source 16 does not supply energy to the secondary power source 12.Instead, a portion of the amount of the available stored energy is usedto power the secondary power source 12 (step 302). During this time, theengine is either on and idling, or, alternatively, off (step 303).Determining whether to idle the engine 16 or to turn it off is a designchoice, and both options provide certain advantages.

If it is desired to maximize the vehicle's 10 drivability, the engine 16remains on and is at idle. This minimizes the driver's perception thatthe engine 16 is no longer generating energy and allows the engine 16 toquickly re-engage when needed. If it is desired to maximize thevehicle's 10 fuel efficiency, the engine 16 is turned off as soon as theavailable stored energy exceeds the first selected level (e.g., entersinto range 36). However, if the engine 16 is turned off too quickly,there is a risk that customers will perceive that the vehicle is losingpower. Thus, to maximize the vehicle's fuel efficiency and furtherminimize drivability disruptions, rather than turning the engine offwhen the available stored energy exceeds the first selected level, theengine is turned off when the available stored energy exceeds the firstselected level and a command to decelerate the vehicle 10 is issued.This provides a more moderate approach that still results in fuelsavings during the time the engine 16 is off, but synchronizes thetiming of engine shut down with a driver issued command. In this way,the driver is able to logically relate to the sensation that the engine16 is no longer generating power with a command that is intended to slowor coast the vehicle.

However, if the available stored energy is below the first selectedlevel (depicted as line 37 in FIG. 2, and at step 301 in FIG. 3), thenthe engine 16 is operated to generate an amount of primary power (step304), and the secondary power source is powered with a portion of directinput energy converted from the primary power source (step 305).

When primary power is generated and the amount of direct input energy issufficient enough to meet a power demand, then direct input energy aloneis used to power the secondary power source, and there is no need to useany of the available stored energy to power the secondary power source.When primary power is generated and the amount of direct input energy isnot sufficient enough to meet a power demand, available stored energymay also be used, together with the amount of direct input energy, toaugment the shortage.

However, in the event that the amount of available stored energy withinthe secondary energy storage device 14 is ever drawn to a level below apreselected “safety” level selected to indicate that the availablestored energy is at or near depletion, it is preferred to discontinueuse of any available stored energy. This is to preserve the life of thesecondary energy storage device 14 and minimize performance problemsthat may result if the secondary energy storage device 14 is operated attoo low of an energy level.

How The Primary Power Source Operates When Generating An Amount ofPrimary Power

How the engine 16 is operated when generating an amount of primary poweralso depends on the amount of available energy within the energy storagedevice 14. The engine's 16 operation is discussed with continuedreference to the exemplary embodiment shown in FIG. 2, reference to theexemplary power efficiency map shown in FIG. 4, and reference to thelogic flow diagram shown in FIG. 5. As will be understood by one ofordinary skill in the art, the curved lines shown in FIG. 4 representthe percent efficiency at which a particular engine can be operated.

In accordance with one embodiment of the present invention, the engineis operated at different power levels depending on whether the availablestored energy is within a predetermined upper range (see FIG. 2, range38), a predetermined middle range (see FIG. 2, range 40), or apredetermined lower range (see FIG. 2, range 42).

When the engine 16 generates primary power (FIG. 5, step 501) and theamount of available stored energy is within the predetermined upperrange of available energy (e.g., below line 37 and within range 38 inFIG. 2, and at step 502 in FIG. 5), the engine is operated at or near apredefined minimum power level for efficient operation of the engine(FIG. 4, point A; FIG. 5, step 503). A first portion of the amount ofprimary power is converted into an amount of direct input energy anddirectly supplied to the secondary power source 12. Directly using thefirst portion of the amount of primary power, as opposed to firststoring the first portion of the amount of primary power and then usingit, as is done in many conventional series hybrid vehicles, serves tominimize energy transfer losses. As a result, greater energyefficiencies result.

In operating conditions where the direct input energy is not sufficientenough to power the secondary power source 12 and meet the driver'spower demand, secondary energy stored within the vehicle's 10 energystorage device 14 is used to augment the required secondary energy.However, if the direct input energy is sufficient enough to meet thedriver's power demand, the engine continues to be operated at or nearits predefined lowest point of power efficiency (FIG. 4, point A) andany additional power generated by the primary power source 16 isconverted into an amount of storable energy for use at a latertime—provided that secondary energy storage device 14 has sufficientenough capacity with which to store the amount of storable energy.

By purposefully having the engine 16 operate at or near its point oflowest power efficiency (FIG. 4, point A) during the time the availablestored energy is within the predetermined upper range 38 (FIG. 5, step502), the primary power generated by the engine 16 is not likely to besufficient enough to power the secondary power source to meet thedriver's power demand. Therefore, it is more likely that an amount ofavailable stored secondary energy will also be used. Thus, although theengine is operated at or near the engine's 16 predefined minimum powerlevel for efficient operation of the engine 16 (FIG. 4, point A),several advantages result.

First, using the vehicle's available stored energy creates anopportunity to use “free” energy (i.e., stored braking energy). Use ofthis “free” energy contributes to the overall energy efficiency of thevehicle. It also creates more space within the energy storage device 14with which to capture more of the vehicle's 10 kinetic energy during thevehicle's next braking event. Second, using the vehicle's 10 availablestored energy minimizes the likelihood that the available stored energywill, within a short time period, repeatedly operate above and below thefirst selected level of available stored energy for a given vehiclespeed. If this were to happen, it could cause the engine 16 to rapidlycycle on and off and result in drivability issues. Third, using thevehicle's 10 available stored energy increases the likelihood that theavailable stored energy will drop to a level within the predeterminedmiddle range 40 shown in FIG. 2.

When the available stored energy is within the predetermined middlerange (e.g., within range 40 in FIG. 2; and at step 504 in FIG. 5), theengine 16 is operated at a range that is near a predefined range ofpower levels for efficient operation of the engine (e.g., between powerlevels B and C on torque/curve line 44 in FIG. 4; and at step 505 inFIG. 5). In one embodiment, the engine is operated within the predefinedrange of power levels, at a rate that is inversely proportional to theavailable stored energy within the predetermined middle range. Forexample, when the available stored energy is at a top most valueselected to define the middle range of stored available energy 40, theengine is operated at a power level that is at or near the low end ofits range of best power efficiency B, and when the available storedenergy is at a bottom most value selected to define the middle range ofstored available energy 40, the engine is operated at a power level thatis at or near the high end of its range of best power efficiency C.Further, when available stored energy is at a value of equal distancebetween the top most and the bottom most value of the predeterminedsecond range, the engine is operated at a power level that is near themidpoint of its range of best power efficiency. Operation of the engineat a range of power levels near the range of power levels B and Crepresents the power level at which the engine is likely to obtain itsbest operating efficiency. Thus, it is desirable to keep the vehicleoperating within this predefined power range as much as possible.

One strategy for increasing the likelihood that the engine 16 will beoperated within the desired predefined power range discussed above is tostrive to maintain the amount of available energy within the energystorage device at a level that is within the predetermined middle range40. Thus, in cases where the amount of available energy stored withinthe energy storage device 14 drops within the predetermined lower rangeof available energy (e.g., within range 42 in FIG. 2, and at step 506 inFIG. 5), including at the lower limit, the engine 16 is operated at ornear a predefined maximum power level for efficient operation of theengine (FIG. 4, point D; FIG. 5, step 507). This causes the engine 16 toproduce more power, and increases the likelihood that the amount ofpower generated by the primary power source and converted into energywill exceed the amount of direct input energy required to power thesecondary power source. When the amount of energy generated by theengine 16 exceeds the amount of energy needed to power the secondarypower source 12, any excess energy is converted into storable energy andstored within the energy storage device. This serves to replenish theamount of available stored energy within the energy storage device andthereby increases the likelihood that the amount of available energy isonce again within the desired predetermined middle range 40.

Under this strategy, even when the vehicle's power demand is not large,if the available stored energy level is within the lower range, theengine will nevertheless continue to be operated at or near its maximumefficient power level until the available stored energy level isrestored to the middle range.

Because the engine must supply the vehicle's full power demand in theevent the energy storage device has been depleted to the lower limit ofthe lower range, the engine is preferably sized to be able to meet suchpotentially necessary sustained vehicle power demands. For example, theengine preferably would be of sufficient size to enable the vehicle toascend a long grade at acceptable speed when fully loaded. Althoughoperation of the vehicle under a condition such as ascending a longgrade occurs rarely, the ability to handle such conditions isnevertheless likely required for a vehicle to be commercially acceptableto the public. An engine that could provide at least 60% to 70% of thedesired peak acceleration power level for the vehicle would likely besufficient for this preferred capability. This preferred engine size inthe present invention differs from prior art series hybrid systems,which instead utilized engines of smaller size.

As discussed previously, many first generation secondary energy storagedevices, such as early generation batteries, required low charge ratesin order to preserve the life of the energy storage device. As a result,in series hybrid vehicles (i.e. wherein the charge rate is provided bythe engine), the engine would need to produce power levels sufficientlylow to generate the required low charge rates for the energy storagedevice. However, use of an energy storage device that can charge atfaster rates for sustained periods of time will allow efficient use of alarger engine in a series hybrid vehicle. It is therefore alsopreferable in the present invention to use an energy storage device thatcan charge efficiently at faster charge rates, which will thereforeenable efficient use of a larger engine size, allowing the engine to runat a high rate of power while the secondary energy source rapidly storesenergy, such as would be preferred for conditions where the availablestored energy is within the lower range as set forth above. An exampleof an energy storage device with the current capability to sustain suchhigher charge rates as are preferred for the present invention is thehigh pressure hydraulic accumulator.

For additional drivability benefits, another preferred embodiment of thepresent invention also considers the power demanded by the vehicledriver in determining the power level to operate the engine, withadditional reference to the logic flow diagram shown in FIG. 7. If theavailable stored energy is within a predetermined middle range (FIG. 2,range 40 and Step 504 in FIG. 7), the engine is still operated within apredetermined range of power levels for efficient operation (FIG. 4,points within B to C; Step 505 in FIG. 7), but the power level of theengine responds directionally (the rate of change is a calibrationdetermination) to the power demanded by the vehicle driver (Step 508 ofFIG. 7). If the driver power demand is greater than the instant powerlevel of the engine, the engine power would be increased (Step 509); iflower, the engine power would be decreased (Step 510). If the availablestored energy is within a predetermined lower range (FIG. 2, range 42,and Step 506 in FIG. 7), a determination is made whether the driverpower demand is greater than the instant power level of the engine (Step511). If the answer to Step 511 is yes, then the engine operatesaccording to Step 507. If the answer to Step 511 is no, then the enginepower level would be reduced (again the rate of change is a calibrationdetermination) in Step 512, but the engine will still be constrained tooperate within the power range of Step 505. An energy storage devicethat could efficiently sustain a charge rate matching 20% to 25% of theengine's maximum rated horsepower (or approximately point A in FIG. 4)would likely be sufficient for this preferred embodiment. Thisembodiment avoids the undesirable noise and feel to the driver ofrunning the engine hard unnecessarily and out of synch with driver powerdemand. It also provides for more efficient engine use in the rechargingof certain secondary energy storage devices.

As will be understood by those of ordinary skill in the art, the size ofthe secondary energy storage device will vary according to vehicleneeds. Factors influencing the size of an energy storage device include,vehicle size, vehicle weight, vehicle speed, and the size of the primaryand secondary power sources. Thus, although the present inventionmonitors available stored energy and performs certain functionsaccording to the level of available stored energy within an energystorage device 14, the precise levels and ranges of available storedenergy chosen as trigger points are a design choice determined by thesefactors. For example, larger energy storage devices 14 will allow thedesigner of a hybrid vehicle to use more of the vehicle's 10 availablestored energy before reaching the threshold level of available storedenergy which triggers the use of primary power, and when primary poweris being used more of the operation (time) will occur within the desiredpredetermined middle range 40.

Optional Use of Additional Primary Power Sources

In an alternative embodiment, multiple engines are used. (As used here,“multiple engines” can also refer to a variable displacement engine witha first engine being the base displacement of the variable displacementengine and “additional engines” being the adding of increaseddisplacement of the variable displacement engine.) For example, when theengine 16 is operated at or near the predefined maximum power level andthe amount of primary power generated by the engine 16 and convertedinto energy does not exceed the amount of direct input energy requiredto power the secondary power source 12, additional engines (e.g., atotal of two or more engines) may be used. Any additional engines may beoperated at or near 1) a power level within a predefined range of powerlevels that is inversely proportional to the amount of available storedenergy within the predetermined lower range (e.g., within range 42 inFIG. 2), or 2) a predefined maximum power level for efficient operationof the second engine.

Further use of additional engines (e.g., a total of two or more engines)may also be desirable when a first engine 16 is operated within thepredetermined middle range of available stored energy (e.g., withinrange 40 in FIG. 2) and the power demanded by a user exceeds apredetermined level. This will lessen the likelihood that the amount ofavailable stored energy will drop into the predetermined lower range ofavailable stored energy 42, and increase the likelihood that the firstengine 16 will continue to be operated within the predefined range forefficient operation of the engine 16.

Additional Embodiments For Triggering Use of The Primary Power Source(s)

Each of the embodiments described above for determining when to use theprimary power source, and how to operate the primary power source whenit is used, is based on the amount of available stored energy. However,in another embodiment, similar to each of the embodiments discussedabove, and illustrated in FIG. 6, rather than basing the determinationsof when and how to generate primary power solely on the amount ofavailable stored energy, these determinations are made based on theamount of available stored energy as a function of vehicle speed. Forexample, to determine whether or not to use primary power to power thesecondary power source, both available stored energy and vehicle speedare monitored and, if the amount of available stored energy for a givenvehicle speed is above a first selected level of available stored energy(depicted by the points comprising line 37 a in FIG. 6), the secondarypower source 12 is powered with a portion of the amount of availablestored energy and not the primary power source 16. However, if theavailable stored energy for a given vehicle speed is below the firstselected level (again depicted as line 37 a in FIG. 6), then the engine16 is used to generate primary power.

In this embodiment, how the engine 16 is operated when generating anamount of primary power also depends on the amount of available storedenergy at a given vehicle speed. As with the embodiments describedabove, the engine is operated at either a minimum power level, a rangeof power levels, or a maximum power level, but its operation depends onwhether the amount of available stored energy at a given vehicle speedis within a predetermined upper, middle or lower range, ranges 38 a, 40a, and 42 a, respectively. Since it is more likely that a faster movingvehicle will be braked harder or longer and thus result in a greateropportunity to generate more storable energy than a slower movingvehicle, this embodiment is designed to power the secondary power sourcewith more available stored energy at lower speeds than at higher speeds.Since it is more likely that the vehicle's 10 braking energy at higherspeeds will replenish the greater amount of used energy, this strategy(i.e., providing more capacity for storing braking energy at highervehicle speeds) furthers the vehicle's opportunity to use of “free”energy and provides yet another means of improving the vehicle's 10overall energy efficiency.

As will be understood by one of ordinary skill in the art, many of themethods may eliminate some steps, include other steps, and/or performthe steps in a different order than illustrated. For example, apredetermined level of available stored energy is a calibration designchoice for the restarting of an engine that was shut off when theavailable stored energy exceeded a predetermined upper range. Further,the various embodiments described above can be combined to providefurther embodiments.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

1. A method of operating a series hybrid vehicle having a primary powersource and a secondary power source, the method comprising: selectivelygenerating an amount of primary power from the primary power source;converting a first portion of the amount of primary power from theprimary power source into an amount of direct input energy; powering thesecondary power source directly with the amount of direct input energy;monitoring an amount of available stored energy within an energy storagedevice; generating the amount of primary power from the primary powersource when the amount of available stored energy is below a firstselected level; operating an engine at one of a plurality of preselectedpower levels to generate the amount of primary power; and selecting theone of the plurality of preselected power levels based on the amount ofavailable stored energy.
 2. The method according to claim 1, furthercomprising: turning the engine off when the amount of available storedenergy is above the first selected level.
 3. The method according toclaim 1, further comprising: idling the engine when the amount ofavailable stored energy is above the first selected level.
 4. The methodaccording to claim 1, further comprising: idling the engine when theamount of available stored energy is above the first selected level; andturning the engine off when the amount of available stored energy isabove the first selected level and a command to decelerate the vehicleis issued.
 5. The method according to claim 1 wherein the plurality ofpreselected power levels includes a predefined minimum power level forefficient operation of the engine, and the engine is operated at or nearthe predefined minimum power level when the amount of available storedenergy is within a predetermined range of available stored energy. 6.The method according to claim 1 wherein the plurality of preselectedpower levels includes a predefined maximum power level for efficientoperation of the engine, and the engine is operated at or near thepredefined maximum power level when the amount of available storedenergy is within a predetermined range of available stored energy. 7.The method according to claim 6 wherein the predetermined range ofavailable stored energy comprises a range at or near complete depletionof the available stored energy.
 8. The method according to claim 1wherein a number of the plurality of preselected power levels residewithin a predefined range of power levels for efficient operation of theengine, and the one of the plurality of preselected power levels iswithin the predefined range of power levels when the amount of availablestored energy is within a predetermined range of available storedenergy.
 9. The method according to claim 8 wherein the one of theplurality of preselected power levels correlates directionally to thepower demanded by the vehicle driver.
 10. The method according to claim8 wherein the one of the plurality of preselected power levels is aboutinversely proportional to the amount of available stored energy withinthe predetermined range.
 11. The method according to claim 1 wherein theengine is comprised of a first and a second engine, the first selectedlevel of available stored energy is higher than each of a second and athird selected level of available stored energy, and the second selectedlevel is higher than the third selected level, the method furthercomprising: operating the first engine when the amount of availablestored energy is below the first selected level; and operating thesecond engine when the amount of available stored energy is either 1)below a second selected level and a command to power the vehicle exceedsa predetermined level of power demand or 2) when the amount of availablestored energy is below the third selected level.
 12. The methodaccording to claim 11 wherein when the second engine is operated, theone of the plurality of preselected power levels for the first engine isat or near a predefined maximum power level for efficient operation ofthe first engine and the one of the plurality of preselected powerlevels for the second engine is at or near one of 1) a power levelwithin a predefined range of power levels that is inversely proportionalto the amount of available stored energy within a predetermined range ofavailable stored energy and 2) a predefined maximum power level forefficient operation of the second engine.
 13. The method according toclaim 1 wherein the primary power source is comprised of a variabledisplacement engine, the variable displacement engine having a firstnumber of cylinders defining the first engine and a second number ofcylinders defining the second engine.
 14. The method according to claim1 wherein the primary power source is either 1) an internal combustionengine, or 2) a Stirling engine.
 15. The method according to claim 1,the method further comprising: based on the amount of available storedenergy, selectively powering the secondary power source with either 1) aportion of the amount of available stored energy, 2) a portion of anamount of direct input energy, or 3) a combination of the portion of theamount of available stored energy and the portion of the amount ofdirect input energy.
 16. The method according to claim 15, furthercomprising: powering the secondary power source with the portion of theamount of available stored energy, instead of the portion of the amountof direct input energy, when the available stored energy is above thefirst selected level.
 17. The method according to claim 15, furthercomprising: powering the secondary power source with the portion of theamount of direct input energy, instead of the portion of the amount ofavailable stored energy, when the available stored energy is either 1)below the first selected level and the amount of direct input energy issufficient enough to meet a power demand, or 2) below a second selectedlevel.
 18. The method according to claim 15, further comprising:powering the secondary power source with the combination of the portionof the amount of available stored energy and the portion of the amountof direct input energy when the available stored energy is either 1)below the first selected level and above a second selected level, andthe amount of direct input energy is not sufficient enough to meet apower demand, or 2) above the second selected level.
 19. The methodaccording to claim 1 wherein the primary power source can provide atleast 60% to 70% of the desired peak acceleration power level for thevehicle.
 20. The method according to claim 1 wherein the energy storagedevice can efficiently sustain a charge rate matching at least 20% to25% of the primary power source's maximum rated horsepower.
 21. Themethod according to claim 1 wherein the secondary power source iseither 1) an electric motor, or 2) a hydraulic motor.
 22. A method ofoperating a series hybrid vehicle having a primary power sourcecomprised of at least one engine and a secondary power source, themethod comprising: monitoring an amount of available stored energywithin an energy storage device; operating a first engine at or near afirst power level when the amount of available stored energy is within apredetermined upper range of available stored energy; operating thefirst engine at or near a second power level when the amount ofavailable stored energy is within a predetermined lower range ofavailable stored energy; and operating the first engine within a rangeof power levels when the amount of available stored energy is within apredetermined middle range of available stored energy.
 23. The methodaccording to claim 22 wherein the first power level is defined by apreselected torque level and a preselected engine speed level, and thefirst power level is a minimum power level for efficient operation ofthe first engine.
 24. The method according to claim 22 wherein thesecond power level is defined by a preselected torque level and apreselected engine speed level, and the second power level is a maximumpower level for efficient operation of the first engine.
 25. The methodaccording to claim 22 wherein the range of power levels comprises anumber of power levels, each of the number of power levels correspondingto a preselected torque level and a preselected engine speed level, andeach of the number of power levels being higher than the first powerlevel and lower than the second power level.
 26. The method according toclaim 22, further comprising: when the amount of available stored energyis within the predetermined middle range of available stored energy,operating the first engine at or near a power level within the range ofpower levels that is inversely proportional to the amount of availableenergy within the predetermined middle range of available stored energy.27. The method according to claim 22 wherein a first selected level ofavailable stored energy is above the predetermined upper range ofavailable stored energy, the method further comprising: idling the firstengine when the amount of available stored energy is above the firstselected level.
 28. The method according to claim 27 wherein a secondselected level is either 1) equal to the first selected level, or 2)below the first selected level, the method further comprising:re-engaging the first engine, following a command to idle the firstengine, when the amount of available stored energy is below the secondselected level.
 29. The method according to claim 27, furthercomprising: turning the first engine off when the amount of availablestored energy is above the first selected level and a command todecelerate the vehicle is issued.
 30. The method according to claim 27,wherein a second selected level of available stored energy is either 1)equal to the first selected level, or 2) below the first selected level,the method further comprising: restarting the first engine, following acommand to turn the first engine off, when the amount of availablestored energy is below the second selected level.
 31. The methodaccording to claim 22, further comprising: selectively operating asecond engine, together with the first engine, when the amount ofavailable stored energy is within the predetermined middle range ofavailable stored energy; and operating the second engine when either 1)the amount of available stored energy is below a selected level withinthe predetermined middle range of available stored energy, or 2) acommand to power the vehicle exceeds a predetermined demand level. 32.The method according to claim 31, further comprising: when the secondengine is operated, operating the first engine at or near the secondpower level, the second power level being a predefined maximum powerlevel for efficient operation of the first engine; and operating thesecond engine at or near either 1) a power level within a predefinedrange of power levels that is inversely proportional to the amount ofavailable stored energy within the predetermined lower range, or 2) apredefined maximum power level for efficient operation of the secondengine.
 33. The method according to claim 31 wherein the engine is avariable displacement engine, the variable displacement engine having afirst number of cylinders defining the first engine and a second numberof cylinders defining the second engine.
 34. The method according toclaim 22, further comprising: selectively operating a second engine,together with the first engine, when the amount of available storedenergy is within the predetermined lower range of available storedenergy; and operating the second engine when either 1) the amount ofavailable stored energy is below a selected level within thepredetermined lower range of available stored energy, or 2) a command topower the vehicle exceeds a predetermined demand level.
 35. The methodaccording to claim 22 wherein the engine can provide at least 60% to 70%of the desired peak acceleration power level for the vehicle.
 36. Themethod according to claim 22 wherein the energy storage device canefficiently sustain a charge rate matching at least 20% to 25% of theengine's maximum rated horsepower.
 37. The method according to claim 22wherein the at least one engine is either 1) an internal combustionengine, or 2) a Stirling engine.
 38. The method according to claim 22wherein the energy storage device is either 1) an accumulator, 2) abattery, 3) an ultracapacitor, or 4) a flywheel.
 39. The methodaccording to claim 22 wherein the secondary power source is either 1) anelectric motor, or 2) a hydraulic motor.
 40. A method of powering asecondary power source in a series hybrid vehicle, the methodcomprising: monitoring an amount of available stored energy within anenergy storage device; monitoring vehicle speed; and based on the amountof available stored energy at a given vehicle speed, selectivelypowering the secondary power source with either 1) a portion of theamount of available stored energy, 2) a portion of an amount of directinput energy, or 3) a combination of the portion of the amount ofavailable stored energy and the portion of the amount of direct inputenergy.
 41. The method of operating a series hybrid vehicle having aprimary power source and a secondary power source, the methodcomprising: monitoring an amount of available stored energy within anenergy storage device; based on the amount of available stored energy,selectively powering the secondary power source with either 1) a portionof the amount of available stored energy, 2) a portion of an amount ofdirect input energy, or 3) a combination of the portion of the amount ofavailable stored energy and the portion of the amount of direct inputenergy; powering the secondary power source with the portion of theamount of available stored energy when the amount of available storedenergy is above a first selected level; powering the secondary powersource with the portion of the amount of direct input energy when theavailable stored energy is either 1) below the first selected level andthe amount of direct input energy is sufficient enough to meet a powerdemand, or 2) below a second selected level; powering the secondarypower source with the combination of a portion of the amount ofavailable stored energy and a portion of the amount of direct inputenergy if the available stored energy is either 1) below a firstselected level and the amount of direct input energy is not sufficientenough to meet a power demand, or 2) above the second selected level;using an engine to generate the amount of direct input energy and togenerate a first amount of storable energy; based on the amount ofavailable stored energy, operating the engine at or near one of 1) afirst predefined power level when the amount of available stored energyis within a predetermined upper range of stored energy, 2) a secondpredefined power level when the amount of available stored energy iswithin a predetermined lower range of stored energy, and 3) a thirdpredefined power level within a range of power levels that is inverselyproportional to the amount of available energy within a predeterminedmiddle range of stored energy.
 42. The method according to claim 41,wherein the first selected level of available energy is above thepredetermined upper range of stored energy, and the second selectedlevel of available energy is below the predetermined lower range ofstored energy.
 43. The method according to claim 41, wherein the firstpower level is lower than the second power level, and a respective oneof each of the power levels within the range of power levels residesbetween the first and the second power levels.
 44. The method accordingto claim 41 wherein the second selected level is at or near completedepletion of the available stored energy.
 45. The method according toclaim 41 wherein the second predefined power level comprises a range ofpower levels correlating to the power demand.
 46. The method accordingto claim 41 wherein the engine can provide at least 60% to 70% of thedesired peak acceleration power level for the vehicle.
 47. The methodaccording to claim 41 wherein the energy storage device can efficientlysustain a charge rate matching at least 20% to 25% of the engine'smaximum rated horsepower.
 48. The method according to claim 41 whereinthe engine is either 1) an internal combustion engine, or 2) a Stirlingengine.
 49. The method according to claim 41 wherein the energy storagedevice is either 1) an accumulator, 2) a battery, 3) an ultracapacitor,or 4) a flywheel.
 50. The method according to claim 41 wherein thesecondary power source is either 1) an electric motor, or 2) a hydraulicmotor.